Fix autocomplete, autocapitalize and autocorrect

New iOS versions doe autocapitalisation, autocrrection and/or autocomplete. All three can be annoying when entering always changing usernames with sometimes lower-case first characters, as in the Captive Portal Vouchers usernames.

This should fix the behaviour and make the portal more graceful towards iOS devices.
(cherry picked from commit 6404281c)

src/opnsense/scripts/OPNsense/CaptivePortal/htdocs_default/index.html

@@ -173,7 +173,7 @@
               <form class="form-signin">
                   <h2 class="form-signin-heading">Please sign in</h2>
                   <label for="inputUsername" class="sr-only">Username</label>
-                  <input type="text" id="inputUsername" class="form-control" placeholder="Username" required autofocus>
+                  <input type="text" id="inputUsername" class="form-control" placeholder="Username" required autofocus autocomplete="none" autocapitalize="none" autocorrect="off">
                   <label for="inputPassword" class="sr-only">Password</label>
                   <input type="password" id="inputPassword" class="form-control" placeholder="Password" required>
                   <button class="btn btn-primary btn-block" id="signin" type="button">Sign in</button>